1. Field of the Invention
The present invention relates to a torque sensor for sensing torque acting on an object, and more particularly to a torque sensor suitable for sensing torque that acts between an input shaft connected to a steering wheel of a vehicle and an output shaft connected to a steering mechanism of the vehicle.
2. Description of the Related Art
Conventionally, a torque sensor of the above-described type has been used in a steering mechanism as shown in FIG. 1.
First, the structure of the conventional steering mechanism will be described with reference to FIG. 1.
A steering mechanism 10 shown in FIG. 1 comprises a hollow shaft 11 connected to a steering wheel (not shown) of a vehicle. A lower portion of the shaft 11 passes through and is supported by an upper portion 12a of a housing 12. Another shaft 13 is inserted into a lower portion 12b of the housing 12. A pinion 14 is provided on a lower portion of the shaft 13, and the pinion 14 is in meshing-engagement with a rack R. An unillustrated motor is provided and drivingly connected to the rack R in order to assist the driver""s steering operation.
A torsion bar 15 is accommodated inside the shaft 11. The upper end of the torsion bar 15 is connected to the shaft 11 by means of a pin 16, and the lower end of the torsion bar 15 is in spline-engagement with the shaft 13.
Therefore, when a torque is applied to the shaft 11 upon operation of the steering wheel, the torsion bar 15 is twisted, so that a relative displacement is produced between the shaft 11 and the shaft 13.
Within the housing 12, a sensor ring 17 formed of a magnetic material is provided on the shaft 11, and a sensor ring 18 formed of a magnetic material is provided on the shaft 13. Further, a torque sensing coil 19 is provided inside the housing 12 such that the torque sensing coil 19 surrounds the sensor rings 17 and 18 with a predetermined gap formed therebetween. When a relative displacement is produced between the shafts 11 and 13, the amount of overlap between the sensor rings 17 and 18 changes, resulting in a change in the inductance of the torque sensing coil 19. Thus, a signal corresponding to the sensed torque is obtained from the torque sensing coil 19. The torque sensing coil 19 is connected to an interface circuit (hereinafter referred to as an xe2x80x9cI/F circuitxe2x80x9d) 80 disposed at the right end of the housing 12 in FIG. 1. The I/F circuit 80 is connected to a microcomputer (not shown) provided in the vehicle.
Next, operation of the I/F circuit 80 will be described with reference to FIG. 2.
DC current supplied from a DC power source 81 is supplied to a regulator circuit 83 via a filter circuit 82, which eliminates unnecessary harmonic components from the DC current. The regulator circuit 83 receives the DC current output from the filter circuit 82 and generates a reference voltage. An oscillator circuit 84 generates a sinusoidal signal on the basis of the reference voltage output from the regulator circuit 83. The sinusoidal signal is applied to the torque sensing coil 19.
As a result, a sinusoidal voltage corresponding to the inductance of the torque sensing coil 19 is generated between the opposite ends of the torque sensing coil 19. The AC component is extracted from the sinusoidal voltage by a DC cut circuit 85 and is detected by a detection circuit 86, so that a DC voltage proportional to the amplitude of the extracted AC component is output from the detection circuit 86. The DC voltage is then input to an addition circuit 87.
The sinusoidal voltage generated between the opposite ends of the torque sensing coil 19 is input to a temperature compensation circuit 88, which outputs a temperature drift signal that indicates variation in the inductance of the torque sensing coil 19 caused by temperature. The temperature drift signal is input to the addition circuit 87.
The addition circuit 87 calculates a difference between the signal output from the detection circuit 86 and the temperature drift signal output from the temperature compensation circuit 88 and cancels out the temperature drift component to thereby output a torque component signal, which is output to a scaling circuit 89. The scaling circuit 89 amplifies the torque component signal at a preset gain to thereby scale up the torque component signal. The scaled-up torque component signal is amplified by an output amplifier circuit 90. Subsequently, after being amplified by the amplifier circuit 90, the torque component signal is output to an A/C conversion circuit 91 as a torque signal, so that the torque signal is converted to a digital signal, which is output to a microcomputer provided in the vehicle.
On the basis of the magnitudes of input digital signals, the microcomputer calculates an amount of assist to be applied to the steering mechanism and outputs to the motor a drive signal corresponding to the calculated amount of assist. Thus, the steering mechanism is assisted through rotation of the motor.
However, the above-described conventional torque sensor requires a large number of circuits, such as a circuit for applying a sinusoidal signal to the torque sensing coil 19 and a circuit for sensing the inductance of the torque sensing coil 19 as torque, which makes it difficult to enhance the reliability of the torque sensor.
Further, in the conventional torque sensor, since the torque signal output from the output amplifier circuit 90 is an analog signal, torque cannot be detected unless the voltage applied to the circuits is prevented from becoming lower than the operation voltage of the microcomputer even when a voltage drop occurs.
Therefore, in addition to a power source for the microcomputer, there must be provided a DC power source 81 which supplies the oscillation circuit 84 with a voltage (e.g., 8V) higher than the operating voltage (e.g., 5V) of the microcomputer.
As described above, the conventional torque sensor is complicated in terms of circuit configuration and requires a plurality of power sources, which makes it difficult to improve reliability and reduce manufacturing costs.
An object of the present invention is to provide a torque sensor which can simplify the configuration of a circuit for sensing torque and which decreases the number of power sources to thereby improve reliability and reduce production costs.
In order to achieve the above object, the present invention provides a torque sensor which is provided with a coil whose inductance changes in accordance with variation in torque acting on an object and which detects the torque on the basis of the inductance of the coil, the torque sensor comprising: a first oscillation circuit for detecting the inductance of the coil and for oscillating a signal having a period corresponding to the detected inductance; a period detection section for detecting the period of the signal generated by the first oscillation circuit; and a torque detection section for detecting the torque on the basis of the period detected by the period detection section.
This structure eliminates necessity for provision of an oscillation circuit for applying a sinusoidal signal to the coil. Further, since the signal oscillated by the first oscillation circuit can be input directly to a microcomputer for direct measurement, digital processing becomes simple, and an A/D conversion circuit or a like circuit becomes unnecessary. Further, since torque can be sensed regardless of variation in supplied voltage, a single power source can be used commonly for torque sensing and for the microcomputer. Accordingly, no additional power source is required.
As described above, the circuit configuration can be simplified and the number of power sources can be reduced, as compared to conventional torque sensors. Therefore, the reliability of the torque sensor is improved, and the production cost of the torque sensor is reduced. Further, since the period of a signal corresponding to a generated torque can be measured directly, a response in detecting the generated torque can be improved.
Preferably, a sensing region of the coil where the inductance of the coil is detected is divided into a plurality of regions; and there are provided a first comparison section for comparing a signal detected from one of the divided regions and a signal detected from a predetermined region, and a layer-short detection section for detecting a layer short of the coil on the basis of a result of comparison performed by the first comparison section. The term xe2x80x9cpredetermined regionxe2x80x9d means one of the divided regions, a plurality of divided regions, or the entire coil.
When a layer short occurs in the coil, the signal flowing through each of the divided regions changes from the value in the case where no layer short occurs, and the layer short of the coil can be detected through detection of the change in the signal.
The first comparison section is preferably configured to detect a first DC component derived from one of the divided regions and a second DC component derived from the predetermined region, and to compare the first and second DC components. In this case, a layer short of the coil can be detected without influence of variation in the signal generated from the coil.
Preferably, a first DC component detection section is provided for detecting a DC component of current flowing through the coil; and the torque detection section is constructed to perform temperature compensation for the detected torque on the basis of the DC component detected by the first DC component detection section. This structure eliminates necessity for provision of a coil for temperature compensation. Therefore, the size and cost of the torque sensor can be decreased greatly as compared with a torque sensor having a temperature compensation coil. In addition, since the DC component of current flowing through the coil is utilized for temperature compensation, temperature compensation can be performed without influence of variation in the signal generated from the coil.
Preferably, the torque sensor further comprises a second oscillation circuit for oscillating and outputting a signal having a period corresponding to the inductance of the coil; a second comparison section for comparing the signal output from the second oscillation circuit with the signal output from the first oscillation circuit; and a first anomalous state detection section for detecting an anomalous state of the first or second oscillation circuit on the basis of a result of the comparison performed by the second comparison section. This structure enables detection of an anomalous state of the first or second oscillation circuit.
In the case where the torque sensor comprises the first DC component detection section, the torque sensor may further comprise a second DC component detection section which is provided for detecting a DC component of current flowing through the coil; a third comparison section for comparing the DC component detected by the first DC component detection section with the DC component detected by the second DC component detection section; and a second anomalous state detection section for detecting an anomalous state of the first DC component detection section or the second DC component detection section on the basis of a result of the comparison performed by the third comparison section. This structure enables detection of an anomalous state of the first or second DC component detection section.
The above-described oscillation circuit is preferably an LR oscillation circuit using the coil and a resistor. Since the LR oscillation circuit does not use a capacitor, the LR oscillation circuit is hardly affected by ambient temperature.
The above-described second comparison section is preferably configured to compare the phase of the signal output from the first oscillation circuit with the phase of the signal output from the second oscillation circuit. In this case, an anomalous state of the first or second oscillation circuit can be detected without influence of variation in power source voltage that serves as noise.